The present invention relates to a sheet for molding a fiber-reinforced resin and a method for producing it and, in detail, relates to a sheet for molding a fiber-reinforced resin which is used as a material when fiber-reinforced resin products consisting of a thermosetting resin reinforced with glass fiber, carbon fiber, etc. are produced by a sheet-molding method such as a vacuum-molding method etc., and relates to a method for producing the sheet.
In the method for producing the fiber-reinforced resin products, there are a hand lay up method, spray-molding method, metal mold-molding method, vacuum-molding method, pressure-molding method, etc., and among these methods a sheet-molding method such as the vacuum-molding method, pressure-molding method, etc., in which a preproduced sheet for molding a fiber-reinforced resin is used, has been developed as a method which is able to produce a moldings of high precision effectively with relatively with relative simplicity.
FIG. 8 shows an outline of a hitherto-known vacuum-molding method, which comprises using a sheet for molding a fiber-reinforced resin S formed in a sheet type by adding a reinforcing fiber such as glass fiber, carbon fiber, etc. into a liquid type thermosetting resin such as a polyester resin etc., setting this sheet for molding S on a female type (or a male type) mold m, shaping the sheet for molding S along a mold face by absorbing it under vacuum from a vacuum absorbing opening V arranged on the face of mold m, then curing thermally the sheet for molding S by heat transferring from the face of mold m, and thus obtaining a fiber-reinforced resin molding having a desired shape.
In the forementioned method, the covering film C consisting of a thermoplastic resin having a soft and stretching character is stuck on a surface not in contact with the mold face of the sheet for molding S. In shaping by the vacuum absorption, this covering film C is used to prevent air-bubble invasion into a resin material of the sheet for molding S, nap-raising of glass fiber, and uneven irregularities forming on the surface, which may be caused by a pressure difference between both faces of the sheet for molding S.
Also, if a fiber-reinforced resin layer containing glass fiber etc. is exposed on a molding surface, because the flat and smooth character of the surface may be damaged by uneven irregularities caused by glass fiber etc. or feeling of the surface may be bad, a surface resin layer consisting of a composition, which is made by adding an appropriate colorant or others into a thermosetting resin not containing reinforcing fiber, is sometimes laminated on the surface being in contact with the mold face of the sheet for molding S.
However, in the hitherto-known methods described above, there is a problem in a process of thermic curing that the covering film C to cover a surface of the sheet for molding S locally melts and, as a result, the covering film C peels off, the air-bubble invasion into the sheet for molding S is not prevented, the smooth surface can not be fulfilled, and the molding finish becomes bad.
That the covering film C locally melts results from a fact that a thermosetting resin composition of the sheet for molding S causes heat generation during thermic curing, the curing proceeds in advance at a surface layer part adjacent to the covering film C, the heat-convection does not take place at a part where fluidity is lost, so that the heat becomes locally full, the covering film C at this part is led to a high temperature and melts. As a result, the covering film C which has been stretched during the shaping peels off in returning to an original condition. At the point in time that the covering film C has peeled off, if the surface of the thermosetting resin is still liquid or under a softened condition, the thermosetting resin may peel off together with the covering film C, and the aforementioned nap-raising of glass fiber, air-bubble invasion, and surface uneven irregularities takes place.
In an effort to solve this kind of problem, the present inventors previously invented a method which comprises curing by ultraviolet rays irradiation, independent of thermic curing of the whole part, of only a thermosetting resin on a surface layer part of the sheet for molding S adjacent to the covering film C, and they have applied for a patent.
FIG. 9 shows an outline of the forementioned method, which comprises the sheet for molding S being laminated, with the fiber-reinforced transparent resin layer a, in which a transparent thermosetting resin combined with a curing agent by ultraviolet rays is immersed into the glass fiber f, and laminated with the surface resin layer b consisting of only a colored thermosetting resin, but not containing the glass fiber f, and also, which comprises irradiating the sheet for molding S, that is shaped along the mold m, by ultraviolet rays from above the covering film C to cure the surface layer part (the part indicated by a cross mark X in the figure) of the fiber-reinforced transparent resin layer a. By doing like this, it is prevented that the heat generated in thermic curing of the whole part of sheet for molding S becomes full on a part of the surface layer part causing a high temperature condition. Also, even if the covering film C locally melts, nap-raising of glass fiber, air-bubble invasion, etc. do not take place when the surface layer part of the fiber-reinforced transparent resin layer a has been cured.
In this method, it is necessary to keep the fiber-reinforced transparent resin layer a quite transparent in order to absorb ultraviolet rays for curing and also, to laminate the colored surface resin layer b with the fiber-reinforced transparent resin layer a in order to produce a colored molding.
However, in the forementioned preceding art there occurs a problem that, depending upon a shape of the fiber-reinforced resin moldings, the surface resin layer b becomes locally thin, the inside fiber-reinforced transparent resin layer a is seen, the surface color tone is not uniformly finished, uneven color occurs, and outside appearance becomes bad.
This is because, in a case where a projecting portion P exists inside the mold face as shown in FIG. 9, the surface resin layer b is locally and strongly pressed or stretched by the projected portion P so that its thickness becomes thin. The surface resin layer b changes the shape more easily than the fiber-reinforced transparent resin layer a does, because the layer b does not contain the reinforcing fiber, and also, since strain in the molding process concentrates on the surface resin layer b, the surface resin layer b becomes thin. As long as the fiber-reinforced transparent resin layer a is colored in a color same to that of the surface resin layer b, the outside appearance does not become prominent even if the surface resin layer b becomes thin, but when the fiber-reinforced transparent resin layer a is colored, ultraviolet rays are not absorbed, so that curing of the surface layer part is not possible by the forementioned ultraviolet rays irradiation.